1. Field of the Invention:
The invention relates to a switchable output circuit, having an output taken from a node between output transistors connected in series for conducting current alternatively, of the type commonly called a totem pole circuit; and more particularly to the coupling between the output transistors of such a circuit and the driver circuits.
Circuits of this type will typically have the output transistors connected in series between two power supply connections, one of which may be ground; or the two supply connections may be at different levels with respect to ground, for example, equal and opposite polarity voltages. Control signals for the output transistors are frequently provided by two different driver circuits, one for each transistor. Where, during normal operation, the output transistors are intended to conduct alternatively, in a switching mode with each being either turned full on or full off, it is convenient to refer to one driver circuit as a pull-up driver circuit where this turns on the output transistor connected to the higher voltage power supply connection; and the other driver circuit may be called a pull-down driver circuit, whose function is to turn on the transistor connected between the output and the lower potential power supply connection.
In designing such circuits, a point of common concern is "cross-conduction" which refers to the situation in which one output transistor is turned on before the other output transistor has turned off. Such cross-conduction has the effect of drawing a high momentary peak current from the power supply, which may be physically damaging to one or more parts of the circuit, or be a source of interfering electrical noise for other parts of the circuitry.
2. Description of the Prior Art
U.S. Pat. No. 4,855,622 discloses a transistor switching circuit having a controlled rate of change of the driver voltages, so that when the input signal changes in a direction to cause switching of the output, the driver circuit which is turning off one output transistor has a controlled change of driving voltage which is substantially completed before the other driver circuit begins such a change of driving voltage. As shown in FIG. 3 of that patent, this control of the pull-down driver circuit is provided by a capacitor C2 connected between the output node of the output circuit and the base connection of the driver transistor Q5.
In the circuit taught in this U.S. patent, the effect of the capacitor C2 is to slow down the switching rate of the lower driver, to prevent its turning on prematurely.